Answer
85 cm
Work Step by Step
Use equation 25-3 to find the magnification of the telescope.
$$M=-\frac{f_o}{f_e}=-\frac{82cm}{2.8cm}=-29$$
See Figure 25-20. When the object and the image are both far away, the separation of the 2 lenses is the sum of their focal lengths.
$$L=f_o+f_e=82cm+2.8cm\approx 85cm$$
